Fireout, TPI? Something, something injection?
My truck had a nice rubber plug behind the passenger side door when I bought it. (you can see it in the pix of my truck). It was about the size of a gas tank filler neck but no way. My neighbor hypothesized that it probably had a CB radio antenna mounted there at one time. I shoulda saved the plug. It would have been perfect for a tank hole plug. I could have patched it by welding in a patch flush like I did the dash radio hole. But, I took the lazy way out and grinded off the paint inside, laid on a piece of fiberglass and then after it dried, filled it outside with body filler. Either way, it's just body work. However, once you do that you cross the line from stock to custom. But what the heck, if you're mounting a custom tank you're crossing over anyways.
Old Gold is absolutely right. If you leave the old filler neck in place, I'd remove it and weld on a cap and then put it back. Anything can happen! I once had an old chevy truck that was born a deisel. When he engine blew I dropped in a big fat Olds gas engine and left the label "deisel fuel only" on the tank doors. One of my buddies who knows nothing about cars borrowed the truck and tried to return it with a full tank. Yup. He pumped in 30 gallons of deisel fuel. My bad.
